Small stones (6)
I believe the answer is:
gravel
'small stones' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gravel that I've seen before include "Driveway surface" , "Stone mixture for paths" , "Small loose stones" , "Coarse sand with small stones used for roads, driveways etc" , "Loose fragments of rock" .)